The Argentine Championship kicked off at the weekend -- but San Lorenzo were not in action.

Their match against Olimpo was postponed to allow them to prepare for the second, decisive leg of the final of the Copa Libertadores, South America's equivalent of Europe's Champions League.

After last week's 1-1 draw in Asuncion, they stand all square at 1-1 with Nacional of Paraguay. A win in front of their own fans on Wednesday night will end a long wait for the continent's premier club prize. Semifinalists in the inaugural version of the Libertadores back in 1960, San Lorenzo are the only one of the traditional Buenos Aires 'big five' who have never won the trophy.

Although they conceded a last-gasp equaliser in the first leg, San Lorenzo are still favourites -- their opponents are a traditional but tiny team who had never before made it out of the group stage in the competition.

The Argentines, though, will have to do it without two of the players who did the most to get them this far. Teenage support striker Angel Correa, one of the brightest talents in the Argentine game, was sold to Atletico Madrid in May -- and his medical tests revealed that he needed surgery on a tumour in his heart, and so he is out of action for the next six months anyway.

The other absentee is Ignacio Piatti, the club's joint top scorer in the campaign, who played his last match for San Lorenzo in the first leg.

Ignacio Piatti, right, celebrates with Mauro Matos after Matos' goal in the first leg. Piatti has since left the club. Gabriel Rossi/LatinContent/Getty Images

He has signed for Montreal Impact and, despite appeals to FIFA, was unable to postpone his move north. Two-footed and intelligent, Piatti will be missed, and his absence will probably lead to a change in the shape of the side.

His slot was on the left side of the midfield quartet. It seems likely that veteran playmaker Leandro Romagnoli will move left instead of operating in his usual role behind the centre forward, Mauro Matos, who scored in the first leg. He could be joined by a second striker, Martin Cauteruccio. If so, coach Edgardo Bauza will have tweaked his side from a 4-4-1-1 to a more attacking 4-4-2 -- which may not displease Nacional's young boss, Gustavo Morinigo.

In common with the best traditions of Paraguayan football, Nacional do not boast a great deal of eye-catching midfield elaboration. But they break forward with pace and purpose. The return from suspension of holding midfielder Marcos Riveros is a real boost to their hopes. With Riveros protecting the back four, fellow midfielders Silvio Torrales and Derlis Orue should have some opportunities to burst into the opposing box -- and a more open San Lorenzo side might increase those chances.

Nacional right back Ramon Coronel is an injury casualty. Much more important to the cause, though, is the condition of centre forward Brian Montenegro, who was forced out of the first leg. He has travelled with the team to Buenos Aires in the hope that he will be fit in time. With Montenegro on the pitch, Nacional are better equipped to get behind the home defence -- especially if San Lorenzo push up high in an attempt to put the Paraguayans under pressure.

Last week, it was Nacional substitute Julio Santa Cruz who made an impact with his late equaliser. But if the pressure keeps rising in the Nuevo Gasometro stadium, Bauza may feel that he has the greater strength in depth in terms of options on the bench.

One is Gonzalo Veron, a quick, skillful striker who suffered a serious knee ligament injury last October, but who is now playing himself back to fitness.

Another is Pablo Barrientos, a highly talented playmaker who began his career with the club over a decade ago and has now returned after years in Europe. With Romagnoli on the verge of moving to Brazil, Barrientos is surely set to become the brains of the San Lorenzo attack -- a process that could take place during the course of Wednesday's game. In both legs of the semifinal and the first leg of the final Barrientos came off the bench to replace Romagnoli.

San Lorenzo boast an excellent home record in the campaign. Six games have brought them five wins and a single draw -- when Chile's Union Espanola struck a late equaliser -- the only goal that the Argentines have conceded in front of their own fans.

Nacional have yet to win away, accumulating three draws and three defeats. One of those draws, though, will give them great heart for the task ahead. In the second round, they travelled to Buenos Aires and came back with a 2-2 draw against Velez Sarsfield -- thus eliminating an Argentine club at least as strong as San Lorenzo.

The same result this time will not be good enough to clinch the title -- the away goals rule is not in operation in the Copa Libertadores final.

If aggregate scores are level, then extra time will be played, followed -- should things remain all square -- by a penalty shootout. If it goes that far, then the San Lorenzo fans will barely be able to take the tension.

Whatever happens in the 55th final of the Copa Libertadores, the Nuevo Gasometro will explode with emotion.